A standard diesel-common-rail injector has an actuator which actuates a needle directly or indirectly (with a servo drive). A piezo-element can also be used as a sensor in order to detect characteristic events, such as for example the closing of the needle. This information can be used to improve the accuracy of the injection processes when controlling the injector.
In the case of a piezo-operated servo injector, the piezo-actuator opens a servo valve by means of the reverse piezo-electric effect, which in turn brings about opening of the nozzle needle by means of the hydraulic connection provided, as a result of which fuel is injected. If the piezo-element is not used as an actuator, it is possible to use it as a force sensor by means of the piezo-electric effect. Specifically, in this context the force which originates from the fuel pressure in the fuel chamber, which is arranged underneath the servo valve, said force acting on the servo valve, can be detected by the piezo-actuator if the piezo-actuator is in contact with the servo valve.
However, in the case of a piezo-servo injector with an air gap (in a non-charged state the piezo-actuator is not in contact with the servo valve here) the piezo-actuator must be charged to a certain extent to make contact with the servo valve. On the other hand, the quantity of charge which is applied to the piezo-actuator must not exceed a certain amount since this could bring about opening of the servo valve and therefore a measurement which changes the needle movement in the injector and therefore the quantity of the injected fuel.